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Tackling the Triple Point

THERMOMETER CALIBRATION
can be performed with a triple-point cell (above), which settles at 0.01 degree Celsius: the unique temperature at which water can exist in its solid, liquid and gas phases all in equilibrium (below). Note that a portion of the crushed ice, which should cover the cell, has been removed to provide a better view of the apparatus.




One of the horrible truths of scientific research is that simple and inexpensive techniques will get you just so far. Beyond some point, increasing accuracy can be obtained only with a disproportional rise in expense, sweat and frustration. That's partly because accurate measurements require an extremely well calibrated instrument, and providing such an exact scale can be a vexing challenge.

Consider thermometers. You might think they would be easy to calibrate: just determine what they read at two known temperatures, like the boiling and freezing points of water. But it's not so simple. These temperatures cannot be reproduced accurately, because they depend on factors that are difficult to control, like atmospheric pressure. For precise work, researchers must resort to more sophisticated techniques.

One method is based on a wonderfully repeatable property of water: the unique temperature, called the triple point, at which water can exist with its solid, liquid and gas phases all in equilibrium. To reproduce this temperature, defined to be exactly 0.01 degree Celsius, researchers rely on a special Pyrex flask filled with ultrapure water, evacuated with a vacuum pump and then hermetically sealed with a blowtorch. At $1,000 apiece, such ¿triple-point cells¿ are beyond the budgets of most home laboratories.

But that is about to change, thanks to George Schmermund, a gifted amateur scientist in Vista, Calif. His device remains within about 0.0001 degree C of the triple point for days and costs less than $50 to build.

The cell is simple to construct. Start with a Pyrex straight-walled flask about five centimeters (two inches) in diameter and at least 17 centimeters (seven inches) long. Schmermund hires a glassblower to thicken and angle the opening slightly for a snug fit between the flask and a large rubber stopper. Without these modifications the lip can shatter explosively. As a precaution, wrap the top two centimeters of the flask with electrical tape.

Drill a hole in the stopper and insert a long Pyrex test tube so that it reaches to within two centimeters of the bottom of the flask. Then hermetically seal the joint with silicone cement. To ensure a tight fit between the stopper and the flask, spread a thin film of silicone vacuum grease uniformly around the bottom two thirds of the stopper.

Although professional units contain ultrapure, triple-distilled water, Schmermund has discovered that ordinary distilled water from a grocery store works just fine. Fill the flask until the water comes to about five centimeters below the stopper when assembled.

Next, you must remove air from the chamber atmosphere as well as any gases dissolved in the water. Schmermund eliminates the need for a vacuum pump by simply boiling the water--the expanding steam will force out the air molecules. First, though, to prevent the water from boiling too violently, shatter a clean test tube inside a towel and drop a few shards into the flask to act as nucleation sites for the forming bubbles. Then, secure the cell in a ring stand and gently rest the stopper on top of the flask to allow the steam to escape.

Heat the flask's bottom with a propane torch until the water boils gently. Dissolved gases in the flask will form visible bubbles on the inner test tube. Keep the water boiling until the convection currents have swept them away and until you no longer see any condensation inside at the top. The condensation will disappear when the internal atmosphere has been completely replaced by hot steam.

Remove the flame and quickly press the stopper down to form a vacuum-tight seal. Before doing so, be sure to protect your hands and arms by wearing long sleeves and a pair of hot-water gloves used by professional dishwashers. Also, hold a towel against the flask.

If you immerse the hot, tightly sealed cell in a cool bath, the water inside the flask will boil again. This delightful effect occurs as the water vapor within the cell condenses, lowering the internal pressure, which then decreases the boiling temperature. When the cell cools completely, you should test the quality of the vacuum by giving the cell a gentle vertical shake. (Be careful, because a vigorous jolt could shatter the glass.) You should hear a sharp ¿snap¿ caused by the so-called water-hammer effect: the water, uncushioned by air, will slam full-force into the glass. If you don't hear the sound, regenerate the vacuum.

TRIPLE-POINT CELL
is a vacuum-tight flask containing water vapor, liquid water and an ice mantle that has formed around an inner test tube.

To reach the triple point, first chill the cell overnight in a refrigerator. Next, you'll need to form a thick ice mantle around the inner test tube. Professionals usually pour a frigid mixture of dry ice and alcohol into the inner well, but Schmermund gets fantastic results with liquid nitrogen, which is much colder. You'll find both refrigerants at your local welder's supply store. Before you add the coolant, dry the inner surface of the test tube thoroughly because the glass could crack if ice forms inside the well. Keep in mind that refraction will make the ice mantle appear to grow faster than it actually does. When the mantle looks like it is nearly touching the flask, dump out the remaining refrigerant.

Using liquid nitrogen entails a complication. The ice mantle will form fastest at the bottom where it is in contact with the nitrogen for the longest time. To make the mantle more even, Schmermund periodically lets all the nitrogen boil away and then drops in progressively longer wooden dowels. Additional nitrogen boils energetically around the dowel, and the expanding gases tend to keep the coolant above the dowel's top.

Separate the mantle from the test tube by filling the well with distilled water and 10 percent isopropyl alcohol to melt the mantle's inner surface. Don't be alarmed, though, when the ice cracks violently. If when you rotate the flask the mantle stays put, the ice is no longer stuck to the glass. When that happens pour off enough of the water-alcohol mixture so that its level is two centimeters below the top of the ice.

Last, place the cell inside an insulated drink container filled with crushed ice and water. Because the ice mantle is buoyant, it will press up against the bottom of the inner test tube, making this spot slightly colder than the triple point. A cutting from a pencil eraser makes an ideal spacer. Rest the thermometer on top of the eraser cutting inside the well. In about an hour, the thermometer will settle on the triple point.

To determine temperature, the best thermometers work by measuring the resistance across a thin platinum wire. Because the change in resistance caused by a given temperature difference is well known for platinum, the triple point is all you'll need to calibrate the instrument. Sadly, such thermometers are very expensive. But Schmermund has an answer for that too, as you'll see in my next column.

Illustrations by DANIELS & DANIELS
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